1. Field of the Invention
The present invention relates generally to a pattern transfer process, and more particularly to a pattern forming method with improved processing precision for a to-be-processed film on which a pattern is to be transferred, and a method of manufacturing a semiconductor device using a pattern formed by this pattern forming method.
2. Description of the Related Art
In general, in a lithography step in a series of steps of a semiconductor device manufacturing process, a resist pattern is used as a mask for dry etching in order to process a to-be-processed film lying under the mask. The resist film is thus required to have high performance in both resolution and resistance to dry etching. In recent years, special attention has been paid to a F2 laser as one of next-generation lithography techniques. The F2 laser, however, has a very short wavelength, and it is very difficult to develop a resist film having a high light transmittance for such a very short wavelength. In this situation, in order to increase the light transmittance, there is no choice but to decrease the thickness of the resist film. If the resist film is thinned, however, the resistance of the resist film to dry etching decreases, and it becomes difficult to achieve high performance both in the resistance to dry etching and the resolution.
Further, in a resist pattern forming method using a low-acceleration electron beam instead of a laser, the distance of transmission of an electron beam in the resist film is short. It is thus difficult to increase the thickness of the resist film.
A solution to these problems is a multi-layer resist process. There are various methods for the multi-layer resist process. In particular, there is an effective method wherein a dry-etching-resistant material is buried in a resist pattern, and the buried material is used as a mask in transferring a pattern onto the underlying film. This method is very hopeful since the resist film is not required to have dry-etching resistance at all, and research and development of resist films can be focused on enhancement of resolution.
Jpn. Pat. Appln. KOKAI Publication No. 7-135140, for instance, discloses an invention wherein SOG (Spin on Glass), which is a coat-type silicon material, is coated on a resist pattern, and the SOG is etched back until an upper part of the resist pattern is exposed.
The above-described method using the buried material, however, has the following problem.
FIG. 16A to FIG. 16C show cases where buried resist films used as buried material are thin. FIG. 16A is a cross-sectional view showing a resist pattern formed as a so-called line and space pattern (L/S pattern) 101a. FIG. 16B is a cross-sectional view showing a resist pattern formed as a so-called isolated line pattern (iL pattern) 101b. FIG. 16C is a cross-sectional view showing a resist pattern formed as a so-called isolated space pattern (iS pattern). This is similar to FIGS. 17A to 17C and FIGS. 18A to 18C to be described later. In cases where buried resist films 102 are thin, as shown in FIGS. 16A to 16C, the step coverage of the buried material is not sufficient, so some step height remains on the surfaces of buried resist films 102 in accordance with the shapes of resist patterns 101a to 101c. 
FIGS. 17A to 17C depict the states near resist patterns 101a to 101c after etch-back of the thinly buried resist films 102 under the condition, “resist film etching rate>buried resist film etching rate.” It is empirically understood that in general, a necessary etch-back amount (depth) of the buried resist film 102 is greatest at the residual part of the iS pattern 101c of the three patterns. However, if etch-back is performed so as to meet the required etch-back amount of the residual part of the iS pattern 101c, the thickness of the buried resist films 102 at the space portions of the L/S pattern 101a and iL pattern 101b becomes smaller than necessary, or reduces to zero. This would pose a problem in controlling processing dimensions when an underlying film 103 is processed.
An effective solution to the above problem relating to the step coverage of the buried resist film 102 is to increase the thickness of the buried resist film 102. FIGS. 18A to 18C show the states near resist patterns 101a to 101c in this case. The buried resist film 102 is formed to have a large thickness. Thereby, regardless of the shapes of resist patterns 101a to 101c, the surface of the buried resist film 102 is flattened substantially completely. In this case, however, it is difficult to control the in-plane uniformity in etch-back depth. In addition, if the thickness of the buried resist film 102 is increased, the process time for the etch-back increases. In particular, as regards the former problem, the tolerable range of variance in in-plane uniformity is narrowed as the thickness of the resist film 104 decreases. This may lead to failure in the application of ordinary etching techniques. The same also applies to the case of using the condition, “resist film (104) etching rate <buried resist film (102) etching rate.”